The present invention generally relates to methods of beta processing titanium alloys. More specifically, various non-limiting embodiments of the present invention set forth herein relate to a methods of processing alpha+beta titanium alloys and near-beta titanium alloys wherein the alloy is subjected to deformation only at temperatures above the beta-transus temperature of the alloy. Other non-limiting embodiments relate to titanium alloys that have been processed in accordance with the disclosed methods.
Titanium has two allotropic forms, a “high temperature” beta (“β”)-phase, which has a body centered cubic (“bcc”) crystal structure, and a “low temperature” alpha (“α”)-phase, which has a hexagonal close packed crystal structure. The temperature at which the α-phase transforms into the β-phase is known as the β-transus temperature (or simply “β-transus” or “Tβ”) of the alloy.
The β-transus of the alloy is dependent upon both the type and amount of alloying elements present in the alloy. For example, alloying elements that are isomorphous with the bcc crystal structure of the β-phase have a tendency to stabilize the β-phase at lower temperatures. That is, these alloying elements tend to lower the β-transus temperature of the alloy, thereby expanding the temperature range over which the β-phase is stable. Such alloying elements are known as β-stabilizing elements or “β-stabilizers”. Generally speaking, the more β-stabilizers a titanium alloy contains, the lower the β-transus of the alloy will be. Examples of β-stabilizers include, but are not limited to, zirconium, tantalum, vanadium, molybdenum, and niobium. See e.g., Metal Handbook, Desk Edition, 2nd Ed., J. R. Davis ed., ASM International, Materials Park, Ohio (1998) at pages 575-588, which are specifically incorporated by reference herein.
In contrast to the β-stabilizers discussed above, alloying elements such as aluminum and oxygen have a tendency to stabilize the α-phase of the alloy and are known as α-stabilizing elements or “α-stabilizers”. That is, these alloying elements tend to raise the β-transus temperature of the alloy, thereby expanding the temperature range over which the α-phase is stable. Generally speaking the more α-stabilizers a titanium alloy contains, the higher the β-transus of the alloy will be.
Titanium alloys are generally divided into different categories based upon the type and amount of alloying elements in the alloy. For example, titanium alloys containing relatively large amounts of α-stabilizers are generally considered to be “alpha alloys” (or “α alloys”). Alpha alloys contain primarily α-phase at room temperature. One non-limiting example of an alpha alloy is Ti-3Al-2.5Sn. The addition of small amounts of β-stabilizers to an α alloy will result in the retention of some β-phase within the alloy. Such alloys are known as “near-alpha alloys” (or “near-α alloys”). One non-limiting example of a near-α alloy is Ti-6Al-2Sn-4Zr-2Mo.
Titanium alloys that contain similar amounts of α-stabilizers and β-stabilizers are known as “alpha+beta alloys” (or “α+β alloys”). Since these alloys have a higher content of β-stabilizers than near-α alloys, they contain more β-phase than near-α alloys. One non-limiting example of an α+β alloy is Ti-6Al-4V. If the amount of β-stabilizers in an α+β alloy is increased, a “near-beta alloy” (or “near-β alloy) can be formed. Near-β alloys generally have microstructures in which the β-phase is the predominant phase in terms of volume fraction at room temperature. One non-limiting example of a near-beta titanium alloy is Ti-5Al-2Sn-2Zr-4Mo-4Cr.
Titanium alloys that contain a sufficient amount of β-stabilizing elements to avoid formation of α-phase on quenching from the β-phase field are known as “beta alloys” (or “β alloys”). Depending upon the amount of β-stabilizers present, a β alloy can be metastable or stable. Metastable-β alloys contain sufficient amounts of β-stabilizing elements to retain an essentially 100% β-structure upon cooling from above the β-transus. However, on aging the metastable-β alloy below its Tβ, α-phase precipitates can be formed. One non-limiting example of a metastable-β alloy is Ti-12Mo-6Zr-2Fe. In contrast, precipitation of α-phase will generally not occur on aging of a stable-β alloy. One non-limiting example of a stable-β alloy is Ti-35V-15Cr.
Since the various titanium alloys discussed above contain different types and amounts of alloying elements, both the processing characteristics and the properties of these alloys generally differ. For example, α alloys and near-α alloys are generally more difficult to work than β alloys at temperatures below the β-transus of the alloy, owing to the relatively low hot workability of the α-phase. Further, α alloys are generally not susceptible to age hardening heat treatments.
In contrast, α+β, near-β, and metastable-β alloys generally have higher ductility than α and near-α alloys and can be age hardened to some degree. However, because the ductility, work hardening and aging responses of these alloy types differ, the processing methods and routes used with one type of alloy may not be useful with another type of alloy. Consequently, it is generally necessary to carefully select the alloy composition and processing conditions to achieve the desired mechanical properties in the final product.
Conventional processing of cast ingots of α+β and near-β alloys to form billets or other mill products typically involves an initial deformation of the material above the β-transus to break up the cast structure of the ingot followed by cooling to a temperature below the β-transus where the α-phase can precipitate within the β-grains. Thereafter, the alloy is typically subjected to an intermediate deformation step at a temperature below the initial deformation temperature, and typically in the α+β phase field of the alloy, to introduce deformation strain energy (or “pre-strain”) into the alloy. A final deformation and/or annealing step above the β-transus to recrystallize the β-grain structure occurs after the intermediate deformation step. After recrystallization, the alloy may undergo additional processing steps, for example forging, typically below the β-transus, to achieve a desired final configuration.
An intermediate deformation step in the α+β phase field is generally considered to be required in order to introduce sufficient strain energy into the alloy structure to drive recrystallization during the final deformation and/or annealing steps. However, during the intermediate deformation step, a variety of defects may be introduced into the alloy. For example, small voids or pores, known as “strain-induced porosity” or “SIP”, may develop in the alloy. The presence of SIP in the alloy can be particularly deleterious to the alloy properties and can result in significant yield loss. In severe cases additional, costly processing steps, such as hot-isostatic pressing, may be required in order to eliminate SIP. Further, because the hot workability of α+β and near-β alloys is relatively poor at the intermediate deformation temperatures, inconsistent deformation may occur within the work piece, resulting in variation in structure and incomplete grain refinement. Additionally, significant yield loss due to surface cracking during intermediate deformation may also be encountered.
Much of the work done on processing titanium alloys has focused on methods of optimizing the microstructure of titanium alloys through control of thermo-mechanical processing steps. Methods for processing ingots of various titanium alloys into billets having a desired microstructure have been disclosed. For example, U.S. Pat. No. 3,489,617 (“the '617 Patent”) discloses methods of processing ingots of an alpha, an alpha+beta, or an “alpha-lean beta” alloy (i.e.,an alloy which contains both α-stabilizers and β-stabilizers but has lesser amounts of β-stabilizers than the α-stabilizers) to refine the beta grain size of the alloy during processing. See the '617 Patent at col. 1, lines 25-29 and col. 2, lines 5-27. The disclosed methods include working an ingot at a temperature above Tβ of the alloy followed by annealing at a temperature at least as high as the working temperature to recrystallize the material, or simultaneously working and recrystallizing the material at a temperature above Tβ of the alloy. Further, according to the '617 Patent, after recrystallization of the beta grain structure, the alloy may be worked from a temperature in the beta field, but it is essential that the major portion of the reduction occur in the alpha-beta field to break up the alpha network. See col. 3, lines 40-53.
Various methods of processing titanium alloy billets into other configurations having a desired microstructure have also been disclosed. For example, U.S. Pat. No. 5,026,520 (“the '520 Patent”) discloses a method of forming fine grain alpha or α+β titanium alloy forgings by isothermally pressing a billet of an α or α+β alloy at a temperature 50° F. to 100° F. above the alloy's Tβ, followed by an isothermal hold at a temperature 50° F. to 100° F. above the alloy's Tβ and preferably equivalent to the forging temperature, and subsequently quenching to arrest grain growth. See the '520 Patent at col. 4, lines 29-58. A second processing step that occurs at the hold temperature and immediately after the holding step and before the quenching step may also be employed. See the '520 Patent at col. 4, lines 59-66.
U.S. Pat. No. 5,032,189 (“the '189 Patent”) discloses processing near-α and α+β alloys by forging a billet of the alloy into a desired shape at a temperature at or above Tβ of the alloy, followed by heat treating the forged component at a temperature from about 4% below Tβ of the alloy to about 10% above Tβ, rapidly cooling to obtain a martensitic-like structure, and annealing the component at a temperature in the range of 10-20% below Tβ of the alloy. See the '189 Patent at col. 2, line 48 to col. 3, line 3. U.S. Pat. No. 5,277,718 (“the '718 Patent”) discloses a titanium alloy billet, and in particular billets of β-stabilized α+β alloys and β alloys, having improved response to ultrasonic inspection where the billet is thermomechanically treated above Tβ of the alloy immediately prior to ultrasonic inspection. See the Abstract of the '718 Patent.
Despite the efforts aimed at improving the microstructure of titanium alloys via thermo-mechanical processing, comparatively little attention appears to have been focused on methods of processing titanium alloys to reduce or eliminate the occurrence of processing related defects, such as SIP. In “Strain-Induced Porosity During Cogging of Extra-Low Interstitial Grade Ti-6Al-4V,” Journal of Materials Engineering and Performance, Vol. 10 (2) April 2001, pp. 125-130, Tamirlsakandala et al. describe investigation of the origin of SIP development during intermediate processing of in extra-low interstitial (or “ELI”) Ti-6Al-4V. In particular, Tamirlsakandala et al. describe establishing constitutive equations and processing maps by subjecting an ingot of ELI Ti-6Al-4V, which was previously deformed by cogging above Tβ and subsequently cooled below Tβ to achieve a lamellar α (i.e., transformed β) microstructure, to various isothermal hot compression tests at temperatures below, near and above Tβ. See Tamirlsakandala et al. at p. 126. Based on this work, the authors suggest introducing a differential temperature into the billet with lower mid-plane temperature and higher surface temperature to avoid formation of SIP during cogging of the alloy. See Tamirlsakandala et al. at p. 130.
U.S. Patent Application Publication No. 2004/0099350 discloses methods of reducing the incidence of SIP in titanium alloys via control of the alloy content.
Accordingly, there remains a need for methods of processing titanium alloys, and in particular, α+β and near-β titanium alloys, that can reduce or eliminate the occurrence of SIP and/or other processing related defects, while still achieving a desired microstructure.